The present invention relates to the measurement of gas flows in industrial processes, particularly processes requiring accurate control of gas flows at rates corresponding to those employed in integrated circuit manufacture.
Accurate control of the mass or volume flow of gasses is essential to many manufacturing operations. During the manufacture of semiconductors and ICs, for example, there are many processing steps which require delivery into an enclosure of a precisely controlled amount of one or more gasses. Generally, the quantity of gas introduced may be controlled by controlling the time of delivery while monitoring mass flow rate. Other manufacturing operations require that the flow rate of each of one or more gasses be maintained at a selected value.
Therefore, a variety of flow meters have been developed for measuring mass flow rates of gasses from levels be 5 standard cubic centimeters per minute (SCCM) to more than 5.times.10.sup.5 SCCM.
In a typical instrument, a small flow is routed through a sensor assembly where the mass flow is measured, while most of the flow is routed through a flow splitter section located in parallel with the sensor assembly. The sensor assembly contains a capillary tube with two resistance thermometers wound on the outside. The resistance thermometers form two legs of an electronic bridge; the other two legs are usually fixed resistors.
Every flow meter design currently in use has some inherent nonlinearity in its response, i.e. the output signal does not vary in a precisely linear manner with flow rate over the entire measuring range. It is already known in .the art to at least partially compensate for such nonlinearity by processing the flow meter output signal in an analog correction circuit which employs one or more variable resistors set to correct for the output signal error at one or more selected points over the flow meter measuring range. Such correction circuits are physically bulky and are difficult to adjust. Moreover, each time there is a change in the nature or composition of the gas which is being conducted through the flow meter, the resistance value of the resistor or resistors must be adjusted. In addition, variable resistors experience drift in their resistance value with time and have a relatively high failure rate.
In other measuring fields, it has been proposed to correct for measurement instrument inaccuracies by converting the measuring signal to digital form, applying the digital signal to a so-called look-up table, deriving a corrected version of the digital signal from the look-up table and then, if necessary, converting the corrected version to analog form. The look-up table is typically a digital memory whose memory addresses correspond to digital versions of respective values of the original measuring signal and whose memory contents correspond to corrected versions of the respective values of the original measuring signal.
While such a system offers the possibility of achieving near perfect measuring accuracy, creation of such a look-up table is both time consuming and costly because the number of corrected values which must be determined and programmed corresponds to the intended measurement precision of the instrument. To cite a simple example, if an instrument has a measurement range of 500 units and is to produce a measurement reading with a precision of 0.5 unit, then 1000 corrected values must be determined and stored in the look-up table.
Moreover, it is generally necessary to derive these corrected values for each individual measurement instrument to take account of variations between even instruments made in the same series.
If an instrument is used to perform measurements on a variety of substances and has a different response to each substance, a set of corrected values must be determined and stored for each substance.
Therefore, this approach to correction of inherent instrument inaccuracies is frequently unacceptable because of the substantial cost which it involves.